keyctl.2: Improvements to KEYCTL_DH_COMPUTE text

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/keyctl.2

@@ -1397,7 +1397,9 @@ Compute a Diffie-Hellman shared secret or public key.
 The
 .I arg2
 argument is a pointer to a set of parameters containing
-serial numbers for three keys used in the Diffie-Hellman calculation,
+serial numbers for three
+.IR """user"""
+keys used in the Diffie-Hellman calculation,
 packaged in a structure of the following form:
 
 .nf
@@ -1411,11 +1413,11 @@ struct keyctl_dh_params {
 .in
 .fi
 
-The
-.IR private ", " prime " and " base
-fields are IDs of the keys, payload of which would be used for DH values
-calculation.
-The result is calculated as:
+Each of the three keys specified in this structure must grant the caller
+.I read
+permission.
+The payloads of these keys are used to calculate the Diffie-Hellman
+result as:
 
     base ^ private mod prime
 
@@ -1437,6 +1439,7 @@ otherwise an error is returned.
 If
 .I arg4
 is specified zero,
+in which case the buffer is not used and
 the operation returns the minimum required buffer size
 (i.e., the length of the prime).